A. Field of the Invention
The present invention relates to a method for the compensation of disturbances upon magnetic measurements on value documents.
B. Related Art
The check of value documents is of great importance, since the value documents are divided, on the basis of the check, for example into value documents which are authentic, forged, or suspicious. Upon the check of the value documents, also the type of value document is ascertained, thus for example in the case of a bank note its currency and denomination. Usually, by means of sensors there are performed magnetic measurements on the value documents, for the purpose of which the value documents to be checked are transported past the sensors and the measurement signals of the sensors for checking the value documents evaluated. Besides bank notes, further value documents such as checks, tickets, coupons, etc. can be checked in the same way.
The value documents have security elements, such as e.g. security strips, security threads, or prints, which contain magnetic or magnetizable materials. For the magnetic coding of a security element, also a certain characteristic sequence of magnetic and non-magnetic regions can be used, the arrangement of which represents a characteristic and machine-checkable coding of the value document, which are ascertained, as described above, by means of sensors, in order to check in this way the magnetic coding.
It has turned out, however, that the check of the magnetic properties of the magnetic materials of the security elements make high demands on the employed sensors, since only small amounts of the magnetic or magnetizable materials are employed, which is why the magnetic fields emanating from the security elements are low upon the check. The measurement and check of the magnetic properties of the security elements by means of the employed sensors is therefore particularly susceptible to disturbances, e.g. electromagnetic disturbances, as they occur for example in the environment of electrical machines